If the object is another phone, it should be easy. You'd want to use a high accuracy GPS in the device, so that would probably be the first thing to start with - getting a GPS to reliably report it's position. Then figure what technology you will use to communicate it's location back to you. Seems like it needs to be low power, you don't want to throw an extension cord along with it, I imagine. Plus something to let the device know it's been thrown and has come to a rest.